1) Field of the Invention
The present invention relates to an image observation optical system. To be specific, it relates to an image observation optical system used in an image display apparatus that can be held on the head or face of an observer and that can be attached to a cellular phone or a portable intelligent terminal.
2) Description of Related Art
In recent years, development has been energetically made for image display apparatuses, specifically for those to be held on the head or face of individuals for entertaining them with a large image. Also, in accordance with recent popularization of cellular phone and portable intelligent terminal, requirements for large view of graphics or text data on these apparatuses have grown.
As conventional image observation optical systems used in image display apparatuses, there are those as disclosed in Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 7-333551 and Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 8-234137. In such an optical system, a prism whose surface having reflecting action has a rotationally asymmetric shape, such as anamorphic, toric or free curved shape, is employed, to compensate image distortion, curvature of field, and astigmatism.
However, if the image display apparatus is designed to achieve high image definition and wide field angle using such a prism, chromatic aberration of magnification by the prism itself becomes large, to cause the problem of imaging performance degradation.